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1. Computer Architecture
Instruction Sets
Characteristics
and Functions

2. What is an instruction set?
• The complete collection of instructions that are
understood by a CPU
• Machine Code
• Binary
• Usually represented by assembly codes

3. Elements of an Instruction
• Operation code (Op code)
—Do this
• Source Operand reference
—To this
• Result Operand reference
—Put the answer here
• Next Instruction Reference
—When you have done that, do this...

4. Where have all the Operands gone?
• Long time passing….
• (If you don’t understand, you’re too young!)
• Main memory (or virtual memory or cache)
• CPU register
• I/O device

5. Instruction Cycle State Diagram

6. Instruction Representation
• In machine code each instruction has a unique
bit pattern
• For human consumption (well, programmers
anyway) a symbolic representation is used
—e.g. ADD, SUB, LOAD
• Operands can also be represented in this way
—ADD A,B

7. Simple Instruction Format

8. Instruction Types
• Data processing
• Data storage (main memory)
• Data movement (I/O)
• Program flow control

9. Number of Addresses (a)
• 3 addresses
—Operand 1, Operand 2, Result
—a = b + c;
—May be a forth - next instruction (usually implicit)
—Not common
—Needs very long words to hold everything

10. Number of Addresses (b)
• 2 addresses
—One address doubles as operand and result
—a = a + b
—Reduces length of instruction
—Requires some extra work
– Temporary storage to hold some results

11. Number of Addresses (c)
• 1 address
—Implicit second address
—Usually a register (accumulator)
—Common on early machines

12. Number of Addresses (d)
• 0 (zero) addresses
—All addresses implicit
—Uses a stack
—e.g. push a
— push b
— add
— pop c
—c = a + b

13. How Many Addresses
• More addresses
—More complex (powerful?) instructions
—More registers
– Inter-register operations are quicker
—Fewer instructions per program
• Fewer addresses
—Less complex (powerful?) instructions
—More instructions per program
—Faster fetch/execution of instructions

14. Design Decisions (1)
• Operation repertoire
—How many ops?
—What can they do?
—How complex are they?
• Data types
• Instruction formats
—Length of op code field
—Number of addresses

15. Design Decisions (2)
• Registers
—Number of CPU registers available
—Which operations can be performed on which
registers?
• Addressing modes (later…)
• RISC v CISC

16. Types of Operand
• Addresses
• Numbers
—Integer/floating point
• Characters
—ASCII etc.
• Logical Data
—Bits or flags
• (Aside: Is there any difference between numbers and characters?
Ask a C programmer!)

17. Specific Data Types
• General - arbitrary binary contents
• Integer - single binary value
• Ordinal - unsigned integer
• Unpacked BCD - One digit per byte
• Packed BCD - 2 BCD digits per byte
• Near Pointer - 32 bit offset within segment
• Bit field
• Byte String
• Floating Point

18. Types of Operation
• Data Transfer
• Arithmetic
• Logical
• Conversion
• I/O
• System Control
• Transfer of Control

19. Data Transfer
• Specify
—Source
—Destination
—Amount of data
• May be different instructions for different
movements
—e.g. IBM 370
• Or one instruction and different addresses
—e.g. VAX

20. Arithmetic
• Add, Subtract, Multiply, Divide
• Signed Integer
• Floating point ?
• May include
—Increment (a++)
—Decrement (a--)
—Negate (-a)

21. Shift and Rotate Operations

22. Logical
• Bitwise operations
• AND, OR, NOT

23. Conversion
• E.g. Binary to Decimal

24. Input/Output
• May be specific instructions
• May be done using data movement instructions
(memory mapped)
• May be done by a separate controller (DMA)

25. Systems Control
• Privileged instructions
• CPU needs to be in specific state
—Ring 0 on 80386+
—Kernel mode
• For operating systems use

26. Transfer of Control
• Branch
—e.g. branch to x if result is zero
• Skip
—e.g. increment and skip if zero
—ISZ Register1
—Branch xxxx
—ADD A
• Subroutine call
—c.f. interrupt call

27. Branch Instruction

28. Nested Procedure Calls

29. Use of Stack